1. Field of the Invention
The present invention relates to an ion source apparatus used in an ion implanter, particularly, the ion source apparatus provided with a cleaning function to remove accretion such as boron, phosphorus, etc. collecting in an ion source chamber and an extraction electrode system, and cleaning optimized method thereof.
2. Related Art
In industries, ion implanting technologies have been generally used for implanting impurities to workpieces such as silicon wafers, glass substrates, etc. when mass-producing products such as integrated circuits, flat panel displays, etc. Conventional ion implanters have been provided with an ion source, which enables to ionize desired dopant elements and accelerating the elements to form ion beams having normal energy.
The ion source includes a single plasma chamber made of graphite, aluminum, etc. and an extraction electrode system extracting ions confined in the plasma chamber. The plasma chamber is constituted of side walls, a top wall and a bottom wall thus forming a house. At the top wall, a feed opening for ion source gas and an introduction opening for filaments or antennas are provided whereas an outlet opening for extracting the ion beam is provided at the bottom wall.
The ion source generates the ion beam by ionizing the ion source gas supplied into the plasma chamber. The ion source gas is dopant gas generally desirable for Diborane gas diluted with hydrogen (B2H6H2), phosphine gas diluted with hydrogen (PH3/H2) or arsine gas diluted with, hydrogen (AsH3/H2).
Ionized processes in the plasma chamber are performed by an exciter (antenna element). This antenna element may be heated filaments or antennas using high frequency (or microwave). The heated filaments emit high-energy electrons as a thermoelectron whereas the antennas applied with high-frequency power generate excitation signals of high frequency (or microwave) with high energy into the plasma chamber.
Either the high-energy electrons or the excitation signals allow giving energy to the ion source gas in the plasma chamber making the ion source gas ionized. The desired dopant gas as the ion source gas may be boron (B), phosphorus (P) and arsenic (As).
The extraction electrode system is provided on the bottom side of the plasma chamber and generally includes 4 pieces of electrodes with holes arranged in parallel. These electrodes are a plasma electrode, an extraction electrode, a suppression electrode and a ground electrode, which are formed on the bottom wall of the plasma chamber. Positive electrical voltage for an ion beam extraction from the extraction electrode is supplied to the plasma electrode whereas positive electrical voltage for an ion beam acceleration from acceleration power is supplied to the extraction electrode. Negative electrical voltage repressing reverse-flow electrons from suppression power is supplied to the suppression electrode. Moreover, the ground electrode is grounded
Following loading of a workpiece such as a wafer, etc. in a process chamber, a vacuum pumping is performed to the process chamber, and then desired gas is introduced into the plasma chamber via a gas introduction opening. Through these processes, electrons emitted from the filaments in the plasma chamber are ionized by colliding with ionizable gas so as to produce plasma In addition, plasma may also be produced by ionizing the ion source gas in the plasma chamber while high-frequency power is provided to the antennas. The ion in the plasma is extracted as the ion beam via the extraction electrode system applied with high voltage. The ion beam thus extracted and deflected by a mass analysis magnet implants ions to the workpiece in the process chamber without conducting a mass separation.
The ion source gas, generally using Diborane gas diluted with hydrogen as described above, is introduced into the plasma chamber so as to extract the ion beam including boron, and then boron ion as a dopant is implanted to the workpiece. Or, the ion source gas may be phosphine gas diluted with hydrogen. With the same process, the hydrogen-diluted phosphine gas is introduced into the plasma chamber, and phosphorus ion as a dopant is implanted to the workpiece.
As discussed, in case the ion beam has been used for extended periods of time, decomposed boron, phosphorus, etc. are apt to accrete to wall surfaces of the plasma chamber thus causing numerous problems. Further, boron, etc. accreting to the wall surfaces re-evaporate due to heat of the plasma which may cause mixture with plasma. Accordingly, because concentration of ion such as boron ion in the plasma becomes more than previously-determined concentration of element such as boron in gas, and its increasing ratio of the concentration is uncertain, implanted amount cannot be well controlled. This problem will be prominent when conducting implantation of a low concentration.
Furthermore, in case not conducting the mass separation, unnecessary ions (for example, phosphorus ion during doping of boron) may be implanted to the workpiece eventually giving negative impact in the treatment of the workpiece. Specifically, when forming semiconductor components onto the workpiece such as wafer, etc., the condition thereof will deteriorate.
Although the above problem can be solved in such a manner that the ion source is frequently disassembled and cleaned, it requires time and cost. Moreover, while cleaning the ion source, the implanter necessarily refrains from operating for a considerable period of time in which to deteriorate the operating ratio thereof.
For solving the above problem, Japanese patent No. 2956412 discloses a cleaning method for an ion source in that H2 gas is introduced into the ion source when not implanting ions to a sample. By this process the ion source will generate hydrogen plasma thus cleaning the interior of the plasma chamber.
Based on the above method, accretion onto the interior of the plasma chamber is removed from the walls thereof by means of heat of the hydrogen plasma, sputtering, etc. Then, the removed accretion is bound to hydrogen so as to produce a hydrogen compound and evacuated outside via the process chamber in a vacuum pumping.
However, the above cleaning method using H2 gas can only apply to the cleaning in the plasma chamber but not well applicable to accretion such as phosphorus, boron, etc. collected over the surface of electrodes. The reason thereto is that hydrogen ion beams discharge hydrogen ion into a process chamber. Thus, in resumed operation of the ion implantation following the cleaning, the ion beams extracting from the ion source contain more amount of hydrogen ion than necessary. Thus, because excessive hydrogen ion is implanted at the same time of dopant, it may deteriorate qualitative characters of semiconductor devices. See Japanese patent Application Laid-Open No. 2000-48734
Moreover, Japanese Patent Application Laid-Open No. 2000-340165 discloses another type of cleaning method. In the method, anode electrodes and interior walls of an ion source are subject to cleaning by means of sputtering with positive ions extracted from plasma This reference discloses gases producing plasma for cleaning, that is, material gases for doping, rare gas such as argon, hydrogen or gases mixed therewith.
But, in the above cleaning methods or any conventional cleaning methods, the cleaning is only applicable to the interior of the plasma chamber of the ion source but not applicable to the extraction electrode system placed outside of the plasma chamber.
The surface of the extraction electrodes collects accretion such as boron, phosphorus, etc. similar to the interior walls of the plasma chamber thus forming insulating layers. These insulating layers easily bond electrons and be electrified, causing direct discharges. Further, when the insulating layers get thicker, they delaminate which gives negative influences to the electrodes. Furthermore, portions where boron or phosphorus accretes produce spatial ununiformity over formation of the ion beams passing through the electrode system. These cause an easy electrification, and also a dielectric breakdown will be caused due to lowering of withstanding pressure over insulators between electrodes.